Sensors for carbon monoxide have been previously proposed and one well known type is based on the; use of a metallic oxide semiconductor, usually tin oxide SnO.sub.2. The basic principle of their operation is the fact that the resistance of a layer of metallic oxide semiconductor changes in the presence of reactants such as organic vapours, carbon monoxide and even water vapour.
U.S. Pat. No. 4,453,151 discloses a sensor for detecting H.sub.2 S which is made from a mixture comprising metal oxides, activators, dopants and binders and which includes a molecular sieve material, e.g. a zeolite, to provide porosity in the product on a molecular scale.
However, previously proposed tin oxide sensors have not been entirely successful in that they have suffered from problems such as lack of sensitivity to very small amounts of the specific gases to be detected, over-sensitivity to surrounding conditions, e.g. humidity, and lack of specificity in operation. They are too readily poisoned by other extraneous gases and have a slow recovery time after an initial detection. The present invention, therefore, aims to provide an improved sensor of increased selectivity that can be used particularly for the detection of carbon monoxide emissions and that, unlike most previous sensors, can also be operated at room temperature.
Accordingly, the invention provides a sensor to detect emissions of gas or vapour, which comprises a substrate having a layer of a composition comprising a semiconductor metallic oxide, a catalyst and a rheological agent to induce porosity into the surface of the layer, the semiconductor metallic oxide being indium oxide or tannic oxide and the catalyst is present in the composition in an amount of from 3 to 30% by weight.
The invention also provides a method of making a sensor.
The substrate, which should be a good electrical insulator, may be, for example, a sheet of glass or ceramic material. A film of the sensor composition from 100 nanometres 1 mm thick is preferably formed on the substrate by applying a paste of the oxide, catalyst and rheological agent in water to the substrate and annealing at a temperature of e.g. from 500.degree. C. to 1000.degree. C. to form a hardened layer.
The proportions of the ingredients of the covering layer composition may be, for example, 3 to 30% catalyst by weight, 5 to 20% by weight theological agent, if desired additives (well known per se) to change the electrical conductivity of the layer, e.g. in an amount of from 0.5-5% by weight; and the balance stannic oxide or indium oxide.
The catalyst is chosen to give specificity of the sensitivity of the sensor to the gas, e.g. carbon monoxide, to be detected. It is preferably platinum, particularly in the form of platinum black, i.e. finely divided platinum, but other catalysts, e.g. palladium, rhodimn, ruthenium, osmium and irridium may be used.
The rheological aid may be chosen from, for example, kieselguhr and sepiolite. It is preferable to use Kieselguhr, (or diatomaceous earth), which is a mass of hydrated silica. It provides an improved open-pore structure surface for the sensing area of the: sensor and increases the available surface area for reaction with the gas to be detected.
To prepare the usable sensor after the oxide layer has been applied to and annealed on the substrate, any suitable means may be utilized to provide electrodes to enable the required resistance measurements to be made. Thus, for example, silver, aluminium or tin electrodes may be formed on the surface of the stannic oxide film after masking desired portions of its surface. The electrodes may be applied by evaporation from a filament or a boat using a conventionally known vacuum system.
The invention is suitable for the manufacture of both so-called "thick film" sensors and "thin film" sensors. In the case of the former, the film thickness is usually from about 1 micron to 1 mm or higher whereas in the latter case it is usually up to about 1 micron.
Thick film sensors of the invention are particularly suitable for use as relatively low temperature devices in ambient temperature environments, and are suitable, e.g., for smoke detection in domestic situations and for personal, portable or fixed gas detectors in for example, coal mines. Thin film sensors, are usually employed as high temperature devices and are suitable, e.g., for process control and environmental monitoring in manufacturing operations involving gas furnaces.
Thick film sensors of the invention are particularly advantageous over known prior art thick film sensors in that they can be employed to operate at ambient temperatures whereas current commercial devices operate at high temperature and so normally require an additional power source, e.g. batteries, to provide the necessary heating.
Sensors of the invention display ohmic resistance and can be connected to conventional resistance measurement means incorporating warning means designed to be activated when the resistance of the sensor changes by more than a predetermined value.
The sensor or at least its chemically-sensitive semiconductor surface, should be housed to avoid light this being conventional practice with sensors of this general type. Thus they can, for example, be housed in housings of the type conventionally used for smoke detectors.
Although not wishing to be limited to any particular theory, it is believed that operation of the sensor is basically as follows. There is a surface reaction which comprises chemisorption of oxygen followed by desorption of oxygen in the presence of, e.g., CO gas. The CO gas is convened to CO.sub.2 during the desorption and electrons are released so that the surface resistance decreases. The increased surface area of the sensor of the invention due to the induced porosity (porosity being the ratio of the volume of void space to the total volume) greatly improves the effectiveness of the sensor. It is also believed that water vapour may play a role in the surface reactions and so its presence, e.g. in normal humidity conditions of up to 90% relative humidity, is needed for maximum sensitivity.
Sensors of the invention have high sensitivity. They can detect less than 10 parts per million of carbon monoxide in air or nitrogen. They are very specific and, as shown below, react markedly to carbon monoxide emissions in contrast to a variety of other gases. They are stable and reproducible, and have long service life in comparison with known similar devices. They can be manufactured commercially at relatively low cost and can be of reduced size relative to comparable devices currently available.
Embodiments of the invention are now described by way of example only.